1. Field of Invention
The present invention relates to an air-fuel ratio control apparatus of an internal combustion engine and control method for the same. In particular, this invention relates to such an apparatus that controls the fuel concentration of an air-fuel mixture in a feedback manner, based on an air-fuel ratio feedback correction value calculated based on the air-fuel ratio of the air-fuel mixture, and an air-fuel ratio calibration value that is sensed from the behavior of the air-fuel ratio feedback correction value, so that the air-fuel mixture is maintained at a target air-fuel ratio.
2. Description of Related Art
A technique for controlling the air-fuel ratio of an air-fuel mixture of the engine to the stoichiometric air-fuel ratio is known for the purpose of purifying the emissions or exhaust gases of the engine using catalytic converters. In this technique, a sensor (e.g., an oxygen sensor) capable of detecting a physical quantity representing the air-fuel ratio is located in an exhaust passage of the engine, and an air-fuel ratio feedback correction value is determined based upon the detected air-fuel ratio. An amount of fuel to be supplied is then calculated based on the air-fuel ratio feedback correction value and the intake air flow rate. Thus, the air-fuel ratio can be controlled by increasing/decreasing the amount of fuel supplied in accordance with the intake air flow rate, depending upon the air-fuel ratio feedback correction value.
In the air-fuel ratio feedback control as described above, "deviations" appear in the air-fuel ratio feedback correction values because of differences among individual engines or individual sensors, or chronological changes, for example. These "deviations" may cause reduction in the accuracy with which the air-fuel ratio is controlled upon transition of the engine from one operating state to another. To overcome this problem, such a deviation that appears in an air-fuel ratio feedback correction value is stored in a memory, for example, and kept as an air-fuel ratio calibration value, which is used along with the air-fuel ratio feedback correction value for controlling the fuel concentration of the air-fuel mixture (as disclosed in, for example, Japanese Patent Laid-open Publication No. HEI 10-220307).
The air-fuel ratio calibration value is input from the behavior of the air-fuel ratio feedback correction value. For example, the air-fuel ratio calibration value is calculated by addition or subtraction, from the positional relationship between the average value of the air-fuel ratio feedback correction values and a specified range, so that the average value falls within the specified range. When the load to the engine varies, the intake air flow rate fluctuates, and the air-fuel ratio changes relative to the target air-fuel ratio. Accordingly, an error is likely to arise in the air-fuel ratio calibration value if air-fuel ratio calibration is performed in this case.
The overall range of the engine load may be divided into a plurality of calibration regions, and an air-fuel ratio calibration value may be obtained for each load region. However, if the engine load fluctuates too rapidly between two calibration regions, an adequate time period may not be available for air-fuel ratio calibration in each load region. Consequently, the air-fuel ratio calibration value may not be obtained, or it may take an undesirably long time until the calibration value is obtained.
If an error occurs in the air-fuel ratio calibration value as described above, the air-fuel ratio control is inaccurately performed during transition of the engine from one operating state to another. Also, if the air-fuel ratio calibration value is not obtained for a long period of time, the air-fuel ratio control during this period is inaccurately performed, resulting in deterioration of the quality of the emissions.
With a vaporized fuel purge system for purging fuel that vaporizes from a fuel tank of the engine into an intake pipe is provided in the engine, the purge concentration needs to be calibrated to the amount of fuel to be supplied, so that the air-fuel ratio can be controlled with high degree of accuracy. However, if the air-fuel ratio calibration value involves an error, as described above, an error also arises in purge concentration reducing the accuracy of control the air-fuel ratio during transition from one operating state of the engine to another. Also, where calculation of the air-fuel ratio calibration value is delayed, purge concentration calibration cannot be started, and the air-fuel ratio is inaccurately controlled, resulting in further deterioration in the quality of the emissions.